Dark Breath

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1:09Back in 2024, on a drizzly summer day, an email landed in my inbox. It was a tip-off about a new study that was about to be published in a big journal. I'm a science journalist. I get sent lots of new research every week. But the subject line grabbed me. It said, evidence of dark oxygen production at the abyssal seafloor. So I opened it and read an extraordinary claim. Oxygen had been discovered two and a half miles down, produced at the bottom of the Pacific Ocean, in complete darkness, in a place where it couldn't possibly be made. School science textbooks tell us that oxygen's made by plants in sunlight, and there's no sunlight at the seabed 4 ,000 metres down.

1:58So this was a tantalising claim. If true, it could upend hundreds of years of scientific knowledge, everything we know about how oxygen, that vital ingredient for life, is made. So I reported on it, as did much of the world's media. Scientists say they've discovered a source of oxygen deep in the Pacific Ocean that they never knew existed. About half of the oxygen in the air we breathe comes from the seas, and until now is thought to be made exclusively by marine plants using sunlight. But a year and a half on, some scientists have serious doubts, and I still have so many questions. Top of that list?

2:33Is dark oxygen real? So I've come back to it, to investigate. And what I've discovered is much more than an interesting scientific claim. I want to tell you about what I've found. It's a story about a scientific surprise. You know, this is like someone finding, you know, anti-gravity, or it was going to be a very controversial thing to get out there. A story of swirling controversy. You don't need to judge whether it's wrong, because it's already from the start impossible. A story that collides head-on with a billion-dollar mission to mine metals in the deep sea. There's enough, basically, to replace every car on the planet with an electric vehicle at the moment.

3:12It's a row about science that became personal. We've been called fraudsters. It's harassment, it's vilification. And a story about how mysterious the deep ocean is. This is Dark Breath. I'm Victoria Gill.

3:29I want to start at the beginning. Meet Andrew Sweetman, Professor of Seafloor Ecology. He's the scientist at the centre of all this. I find him in a building packed with deep sea research equipment at the Scottish Association for Marine Science in Oban, north-west Scotland. Over here we have two of our 13 deep ocean landers. These are what are called seafloor respirometers. And this one has just arrived, actually. I'm quite glad that we're in here and not out at sea today because it's pretty wet out there. Welcome to Scotland in November. Andrew's deep sea landers are basically autonomous chemistry labs that carry scientific equipment to the seabed.

4:10The key bit of equipment on the lander is a chamber that clamps down and encloses a section of the seafloor so that samples and measurements can be taken from inside it. That's what Andrew and his team were using when they first registered a strange signal two and a half miles down on the Pacific seabed back in 2013. So if we just enclosed ourselves in a glass jar and we put a sensor, we would be able to measure the amount of oxygen consumption you or I was doing, and that's what we were trying to do at the seafloor. And when the instrument came back, the syringes were just bubbling. It was like you were looking at a can of very cold soda that had just been shaken and then opened.

4:51I looked at it and I was like, well, that's not right. Why is that not right? Because it was basically showing that the syringe samples were super saturated with a gas. It wasn't CO2. We later found out it was oxygen. That oxygen was being produced? Yes. And what did you think? I thought, I've obviously done something wrong or these samples are compromised in some way.

5:17They expected the exact opposite. it. Oxygen should be declining, consumed by the microbes and tiny creatures that live at the seabed. But somehow it appeared that oxygen was being produced. Andrew Sweetman first saw the phenomenon in 2013. Then he says he saw it again in 2015 and again in 2018. After every cruise, I would send components back to the manufacturer to say, look, these aren't working. I cannot figure out where this oxygen is coming from. And they would send them back a couple of months later, saying everything's fine, we've checked them. Then in 2021, it happened again. Another scientist on that research trip was Andrew's then PhD student, Daniela de Jonga.

5:56So when we got the data on the vessel, I showed him the graphs and Andrew was looking at the screen and he said, oh, that's great, the sensors are working because I can see the curves going down, which is a classical sort of oxygen consumption curve like you would expect. But then I said, no, actually, this is showing that the oxygen is going up. I said to her, OK, throw the sensors in the bin because we just cannot trust these. So she did. Each of these sensors cost about£10 ,000, so that was£30 ,000 in the bin. And then I started using the second method that I'd brought. Basically, what I was doing was chemically determining the amount of oxygen.

6:35So Andrew and Daniela did that second experiment on board the ship. They analysed seawater collected from inside the enclosed chamber. And stick with me here on the details because the method's key. As the chamber sits with its piece of seabed locked inside, the team can set automated syringes to suck out samples of seawater. Each one captures a frozen moment in their seabed chamber experiment. Andrew then took each of those seawater samples and used a method called titration to measure the concentration of oxygen in each one. I just remember that it was getting more silent and silent at the back of the lab, Just seeing him sit with his hand on his forehead, just, I don't know, looking very intense.

7:17There might have been some swearing involved. And suddenly I realised that with two independent measurements, we were seeing oxygen go up. Up until this point, Andrew had dismissed the strange oxygen phenomenon again and again. You saw the oxygen going up and you disregarded that. I disregarded that for about nine years. Why did you disregard it for nine years? Why for you was this so impossible? Because the only way that I knew that oxygen could be produced at that point was through photosynthesis. You know, the clue is in the word photosynthesis, photo, light. And at four kilometres depth, there is essentially no light.

8:04Why after nine years of dismissing it? What changed your mind? Suddenly we had two measurements saying the same thing. So this wasn't a sensor issue. The first thing that I think I said to Danny, without explaining why, I said, can you please take the sensors out of the bin? So Andrew and his team had seemingly made a huge discovery. Oxygen somehow being produced in complete darkness at the seafloor. But they didn't feel jubilant. We tried to think through all these sort of method issues that could potentially be there and try to work through them all. But none of that explained what we were seeing.

8:43Was it a joyous feeling? No. Was it an oh my God feeling? Yes. Was it almost like the weight of the world on your shoulders? Yes, it was an uncomfortable feeling. Because I realized, OK, we're going to be trying to publish this. and it's going to be extremely difficult and it's going to be extremely criticised and it wasn't a good feeling to think about the stuff that this is going to lead to personally. Explain to me why you knew instantly that this was going to really be controversial. We were about to say that, you know, that thing that you learnt in high school and primary school about photosynthesis being the only source of oxygen on the planet, well, that might not be 100 % of the story.

9:40You know, this is like someone finding, you know, anti-gravity, or it was going to be a very controversial thing to get out there. And it was. But even more than the sheer surprise of the finding, there's another element of this story, a bit of context that made the stakes even higher. because the site that Andrew was investigating is special. It's called the Clarion-Clipperton Zone, a huge expanse of seafloor between Hawaii and Mexico that's littered with small black potato-sized lumps that fall naturally over millions of years, and they contain metals. Andrew thinks the apparent production of dark oxygen could have something to do with these lumps.

10:21In this area where we're seeing this production of oxygen at the seafloor, there are abundant metal oxide deposits, manganese oxide deposits, and they're in the form of polymetallic nodules, which are these dark brown-black concretions. You've got one here. It looks like a lump of coal. Yep. It's sort of like a little small potato-sized... Yep, and they just sit at the seafloor. Am I right to pick it up? Yep, pick it up. Are they really heavy? Yep, and they're full of critical metals that go into electronics, car batteries, those types of things. What metals are in that little spud size? Well, there's manganese, there's iron, there's lithium, there's cobalt, there's copper, there's nickel.

11:09So there's enough deposit down there in the clarion-glypidin zone to power the global society for critical metals for decades. Deep-sea mining companies want the metals in these nodules to power the green economy. It's been called a trillion dollar race to mine metals on the seafloor. There's enough basically to replace every car on the planet with an electric vehicle at the moment. That's Michael Clarke, environmental manager for the Metals Company, a Canadian deep sea mining operation. The Metals Company has a permit to explore and do research in the Clarion-Clipperton zone. In the future, they want to mine there.

11:44When Andrew Sweetman made his dark oxygen discovery in 2021, he was working for Michael and the Metals Company. Look, if dark oxygen is real, it is a huge finding. It's a source of oxygen that's never been discovered before, that's not linked to photosynthesis. That is huge. If you're going to make claims like that, you need to have a very strong story. You need to have a lot of evidence that the mechanism that you're proposing is supported by robust evidence. We don't think in any way, shape or form that paper provides enough evidence to make those claims. And they claim that there's more information to add to what was published in Andrew's study.

12:24We actually have access to all the data that was collected during those offshore campaigns. And the paper that was presented in Nature Geoscience omits a lot of information that actually undermines dark oxygen theory. We'll get more into the detail of the scientific debate later, but Michael says that other researchers have studied the Clarion-Clifton zone and found no evidence of dark oxygen being produced. He argues that Andrew Sweetman's claim is simply flawed science. I am very defensive about our science programme. I want this to be very, very high quality science. And when I see people who are associated with us, who we've actually funded, not producing high quality science, I will call that out.

13:07I think that's part of my job. Michael says Andrew Sweetman left the project early and that the metals company then engaged a third party to go back out to the same area with more sophisticated landers. That third party, he says, found no dark oxygen.

13:24The dark oxygen discovery made headlines around the world. And with that global coverage came intense scrutiny from other scientists. Some shared critiques of Andrew and his team online. But what started as a debate about science then took on a darker, more personal tone. I mean, the things that have been said online, said on social media about this study, the group of scientists, we've been called fraudsters. It's gone way beyond that. It's harassment, it's vilification. And, you know, at the end of the day, I'm just a, we are just humble scientists that have found something that doesn't fit the narrative.

14:08And we're just trying to figure out what's going on. Andrew Sweetman and his team drew criticism from scientists and non-scientists alike. Comments have appeared on social media questioning the research and the conclusions, but I've also seen comments about the competence and the integrity of Andrew and his team. Some posts I've read on social media platforms, including LinkedIn and Reddit, have used terms like fabricating science, shoddy and even fraud to describe his claim of dark oxygen production. 99.9 % of the feedback has been really, really positive. And it's that 0.1 % that is exhausting and upsetting.

14:46I think the hardest thing is when my daughters come downstairs after reading some social media about something that has been said about me and then I have to explain that there are people that don't have all the bits of information that they need and that's the dark side of social media. His former employer also weighed in. In September 2024, a post on X from the metals company's CEO, Gerard Barron's account, described dark oxygen as bad science and Andrew Sweetman as having, quote, a dark ego. It went on to say of Andrew that, quote, this wannabe Einstein chose to go public with his unsupported extraordinary claims about dark oxygen production.

15:29A follow up post included a short video of Russell Brand comparing himself to Jesus. We asked the metals company about these posts in an email, but they didn't respond.

15:41I've been a science journalist for a long time, and I've spoken to many researchers who disagree over the conclusions that can be drawn from a set of data. But this row has at times felt hostile. It could be because dark oxygen crashed into a much bigger debate about whether we should mine the deep sea at all.

16:04Now, no commercial deep sea mining has actually happened yet. It's not allowed. And for the Clarion-Cliberton zone, which is in international waters, a company would need a license from the UN's International Seabed Authority, essentially international permission to mine. No company has that yet. But many people want these metals that are sitting untouched on the seabed. Tonight, President Trump signed an executive order that he says aims to speed up deep sea mining. He says the goal is to get more rare earth metals from waters off the U.S. outer continental shelf. That executive order could effectively bypass the U.N.

16:40licensing process I just mentioned. So this could allow for permits to be issued to mine the seabed in areas that President Trump describes as, I quote, beyond national jurisdiction. That is outside U.S. waters. The metals company has already spent hundreds of millions developing technology to suck up deep sea nodules and bring them to the surface. and to study the potential impact of mining. So what do we know about the environmental impact of hoovering up these nodules off the seafloor? Scientists have discovered quite a lot of different kinds of small animals living there. And they've discovered that half of that biodiversity lives directly on the nodules themselves.

17:19So when you remove the nodules, you've removed their habitat. And those nodules take a million or more years to form. That's Professor Lisa Levin. She's a researcher at the Scripps Institution of Oceanography in California in the US. We used to think the deep sea, with its extreme pressures and total darkness, was devoid of life. Decades of research expeditions, some carried out by seabed mining companies, have now shown us that that's not the case. And we're still learning about the deep ocean and about what commercial mining operations could do to it. Here's Michael Clarke, environmental scientist from the metals company again.

17:56It's an extractive industry. There's always going to be impacts. But from a conservation biologist perspective, it makes sense to me to go to the biggest habitat type we have, where there's the least life, because there is less life at the seabed than there is in rainforests in Indonesia, and at least investigate it for mine. Michael's point is that we need these resources, and so they should come from a place where they'll cause the least environmental damage, the seafloor rather than the rainforest. Lisa Levin thinks the claim of dark oxygen production is emblematic of just how little we know about how life in the deep ocean actually works.

18:30It really is one of our last relatively pristine frontiers. I think that we are not yet ready to destroy them until we better understand what it is we would be giving up. That's why scientists like Andrew Sweetman want to study the deep, dark sea floor. Every rock that we turn over, every animal that we find may be a new species. It's a no-brainer to me. I want to work there. So let's get back to Andrew's work and to the science. One of the things I want to understand here is whether dark oxygen is actually possible. How could metallic seafloor nodules be producing it? So this is where things get a bit fuzzy.

19:12We have not ruled out that there's some sort of microbial process going on. The other idea we have is that there may be some sort of water oxidation or electrochemical reaction. It's a hypothesis, and I just want to say that yet again. It's a hypothesis. It needs to be tested, and we are starting to test that now. The oxidation hypothesis is that somehow metallic ingredients in the nodules react with seawater, and that one of the products of that reaction is oxygen, the O in H2O. Or the electrochemical idea. Andrew thinks the nodules themselves might behave like batteries, generating electrical currents that split water into hydrogen and oxygen.

19:55That's electrolysis. That's basically when you put a battery into salt water, you start to get bubbles. That's your oxygen and hydrogen being liberated electrically. Andrew says he doesn't know if this might be occurring naturally or whether it's something to do with the landers themselves stirring up the seabed, triggering some kind of oxygen-making process. And Andrew stresses this is all theoretical, that he hasn't yet worked out how dark oxygen is produced. But that hasn't put off the critics. Several scientists raised concerns about Andrew's conclusions and about his experiments. We asked two researchers about some of their main criticisms.

20:31My name is Per Hall in Swedish, or in English it would be Per Hall, maybe. And I'm a retired professor. My speciality is marine science, especially marine chemistry. Per Hall has been working with deep sea landers for 30 years. I don't believe that he found dark oxygen production. I mean, his data, his results do not show that dark oxygen production takes place on the Abyssal Sea floor of the Pacific. That is not the same thing, that it does not exist, but he didn't show it. What is your key problem with the dark oxygen experiments? The experiments were not conducted in the correct way. There were too poor quality control of the incubations.

21:22So Threatman cannot know that the results they got are correct ones and not due to artifacts. Per means the oxygen could have been accidentally introduced into the experiment to give a false result. He could have been bringing down surface water. There could have been trapped air bubbles, which would then implode, actually, and then create pockets of oxygen-rich water in the chamber, which could then create this oxygen production. And there could be oxygen dissolved in the plastic walls of the chamber, which then slowly diffuses into the water of the chamber. If Perhal is right, then the evidence proposed for dark oxygen production would be no evidence at all, just contamination.

22:10Another scientist we spoke to takes issue with the theories that explain how dark oxygen could be produced. I'm Professor Ángel Cuesta. I'm a professor at the University of Aberdeen in chemistry. And my area of expertise is electrochemistry, is the kind of processes that power batteries, amongst many other things. My opinion is that the explanation they give for the process makes no scientific sense. Angell believes Andrew's hypothesis about the sea nodules, that they could act like batteries and split water into hydrogen and oxygen through electrolysis, is thermodynamically impossible, that it would violate the laws of nature.

22:48I like to compare this with a round stone on the top of a hill. And if you leave that stone loose, it will roll down the hill. But you would never see a stone at the bottom of the hill going uphill. That would never happen. That's the same thing with chemical reactions. And in the case of water, hydrogen and oxygen combining to produce water, that's going downhill. Taking water and splitting it into hydrogen and oxygen, that's going uphill. And then you would need to identify where the ion exchange is coming from. Like Perhal, Angel doesn't discount that there could be dark oxygen production in the deep sea.

23:21But he thinks that Andrew Sweetman's hypothesis for the mechanism behind it is simply wrong. Any hypothesis that you put forward should be, first of all, possible. It might be wrong, but it has to be possible. This hypothesis does not even reach the point of being possible. You don't need to judge whether it's wrong, because it's already from the start impossible. Both Perhal and Angel Quester joined a group of scientists, including some from the metals company, to publish a critique of the dark oxygen discovery in a journal called Frontiers in Marine Science. Back on his boat, in heavy rain, Andrew Sweetman tells me he has two decades of experience deploying and gathering data all over the world using deep-sea landers.

24:01We've been using landers for 20 years. This isn't just one lander. This is multiple landers. There's hundreds of these things that have been sold. They've been deployed hundreds of times. The manufacturer has never seen any problems with bubbles. We've used them many, many times and have never had any problems with bubbles. Andrew says it was only years of repeated experiments that convinced him that this phenomenon was real. I ignored this for nine years and it was only when this data repeatedly kept showing up with multiple instruments. And then I would go off to another area, for example, in the Atlantic or the Arctic using the same equipment and I wouldn't see it.

24:38That I went, well, I'm obviously doing this right. So what's going on? Because it's probably not me and it's probably not the equipment. So the criticism hasn't made you doubt yourself? As a scientist, you will never be 100 % certain, but I'm 99 % sure that this is happening. For Daniela de Jonga, who was a key part of Andrew's research team on that all-important 2021 expedition, the challenge of studying the deep ocean can leave any discovery, especially one as extraordinary as dark oxygen, open to being questioned. The problem with working in deep-sea environments, right, is you cannot just sort of walk up to it and take a look.